The use of dichroic dyes in solution with various liquid crystal materials, and liquid crystal display devices incorporating such mixtures are well known in the art. These dichroic dye-liquid crystal mixtures may be used in "guest-host" type liquid crystal displays or, when an optically active additive is employed, in "phase-change" type liquid crystal displays. In both types of displays the host liquid crystal material has its orientation controlled by the application of electric fields and in turn interacts with the guest dichroic dye to induce a cooperative conforming alignment. The dichroic dye manifests low or high absorbance of incident light depending upon its molecular orientation with respect to the light wave vector. Thus, generally when an electric field is applied to the display, the area between the electrodes appears clear, i.e., all the dyes exhibit minimum absorption, and in the absence of an electric field the display appears characteristically dye colored, i.e., the dyes are in a high absorbing state.
It is desired to provide a liquid crystal display which appears neutral black in the absence of an electric field and clear or non-black in the presence of an electric field. In order to provide such neutral black appearing displays a plurality of dichroic dyes which together absorb strongly throughout the region between about 400 nm and 700 nm are required. Suitable dichroic dyes which have absorption maxima between about 400 and 600 nm are readily found in U.S. Pat Nos. 4,145,114; 4,128,497; 4,179,395 and in Applicants' copending U.S. Ser. No. 70,421. However, suitable dichroic dyes which absorb strongly between 600 and 700 nm are not so easily obtained. While there are available in the prior art dichroic dyes which absorb light having wavelengths in excess of 600 nanometers, they are deficient in one or more properties required for optimal performance in a liquid crystal display, i.e., they have inadequate solubilities in liquid crystal material, they are chemically, photochemically or electrochemically unstable, e.g., they are degraded by ultraviolet radiation, they have relatively low extinction coefficients, i.e., less than 2.times. 10.sup.4, or they have relatively low optical order parameters, i.e., less than 0.65. For example, U.S. Pat. Nos. 3,960,751; 4,154,746; and U.K. patent application GB 2,011,940 A disclose a number of anthraquinone type pleochroic dyes which have absorption maxima very close to about 600 nanometers. However, anthraquinone type dyes have optical order parameters generally less than 0.65. The optical order parameters are measures of the efficiency with which the dichroic dye is oriented by the liquid crystal material and is directly related to the contrast one observes when viewing the device. In general, dyes having optical order parameters close to 1 are desired since they produce guest-host displays having high contrast, i.e. approximately 10 to 1 and greater.
U.S. Pat. Nos. 4,128,497 and 4,145,114 disclose poly(arylazo) type pleochroic dyes having absorption maxima at wavelengths in the range of from about 590 nanometers to about 620 nanometers. While these dyes have optical order parameters in excess of 0.5, in general they lack stability, i.e., they are degraded by ultraviolet radiation, and they have solubilities in liquid crystal materials of less than 1% by weight.
Masanobu Wada and Tatsuo Uchida in "GH-LCDs Make Bright Color Displays Possible," Japan Journal of Electronic Engineering, November, 1979, disclose several substituted benzothiazyl poly(arylazo) dichroic dyes for use in guest-host liquid crystal displays. These dyes have, in general, lower optical order parameters, i.e., less than 0.65, lower absorption between 600 and 700 nm, i.e., less than 0.3, and lower extinction coefficients, i.e., less than 5.times.10.sup.4, than the dyes of the present invention.